Remote Sensing and geophysical Methods for Evaluation of Subsurface Conditions Matt Houston

1. Remote Sensing and geophysical Methods for Evaluation of Subsurface ConditionsMatt Houston

2. Introduction • Remote sensing and geophysical methods encompass a wide range of airborne, surface, and downhole tools which provide a means of investigating hydrogeologic conditions and locating buried waste materials.

3. Introduction • Advantages and disadvantages • There is no universally applicable geophysical method. • Some methods are quite site specific • It is up to the user to select a method or methods and how they apply to specific site conditions and project requirements.

4. Advantage • Unlike other methods, the geophysical methods provide nondestructive, insitu measurements of physical, electrical, and geochemical properties of the natural or contaminated soil and rock.

5. Disadvantage • The success of a geophysical method depends on the existence of a sufficient contrast between the measured properties of the target and background conditions.

6. Background • Traditional approaches to subsurface field investigations at hazardous waste disposal sites have often been inadequate.

7. Background • Site investigations have relied upon direct sampling methods such as • Soil borings and monitoring wells for gathering hydrogeologic data and soil and water samples • Lab analysis of soil and water samples to provide a quantitative assessment of site conditions • Extensive interpolation and extrapolation from these points of data

8. Background • This approach has evolved over many years and is commonly considered a standard. • However, there are many pitfalls associated with this approach, which can result in incomplete or even erroneous understanding of site conditions.

9. Background • The single most critical factor we face in site evaluation work is accurately characterizing the sites hydrogeology. • If an accurate understanding of the sites hydrogeology is obtained then the predicting the movements of contaminants, or designing a clean up operation would be easy.

10. Background • If all strata where uniform and horizontal then 1 monitoring well would do, unfortunately this is not the case in most areas. • In order to accurately sample an area of unknown hydrogeology one would have to reduce the area to swiss cheese, in order to find fractures, channels, sand lenses, and local permeable zones.

11. Sample Density • The area of the target (At) is 1/10 of the entire area (As). So As/At=10

12. Sample Density • With a 100% probability of detection one would have to have 16 monitoring wells. As As/At get larger the amount of wells needed goes up drastically.

13. Sample Density • This is the primary reason for the application of geophysical methods.

14. How Geophysical Methods are Used • GPM measure a large volume of the subsurface. • Anomaly detectors

15. How Geophysical Methods are Used • Once an overall characterization of the site has been made using GPM and anomalous zones identified, a better drilling and sampling plan can be designed.

16. How Geophysical Methods are Used • Locating soil borings and monitoring wells to provide samples that are representative of site conditions. • Minimizing the number of samples, borings, and/or monitoring wells required to accurately characterize a site. • Reduce field investigation time and cost. • Significantly improving the accuracy of the overall investigation.

17. How Geophysical Methods are Used • Much greater confidence in the final results • Fewer borings and wells and lower cost. • Monitoring wells cost 75,000 (1987 dollars) over 30 years. • Smart Holes

18. Imaging Methods • Large Scale aerial photos • Small Scale aerial photos • SLAR (Side Looking Airborne Radar) • Thermal Infrared**

19. Thermal Infrared • Measures the thermal response • Earth’s surface emits radiation in the thermal infrared wavelength. • Fluctuations can help identify springs, seeps from a landfill, moist and dry areas, vegetation stress, can be used to characterize surface soil and rock

20. Nonimaging Methods • Do not result in a picture, but provide a measurement of some parameter along the flight path. • Electromagnetic measurements • Magnetic measurements • Radiometric measurements • Ground-penetrating radar

21. Ground-penetrating Radar • Reflection of radar waves occur when there is a change in the dielectric constant

22. Conductivity and Resistivity Methods • Resistivity and conductivity are inverse of each other. • They are a function of • Type of soil and rock • Porosity • Conductivity of the fluids that fill the pore spaces

23. Conductivity and Resistivity Methods • Anomalies can be detected • Inorganic plume

24. Seismic Refraction and Reflection • Uses seismic velocity of the rock or soil. • Some sort of sound wave is emitted and measured by a geophone as it travels back to the surface.

25. Seismic Refraction and Reflection • Can be used to • Determine the top of bedrock • Depth of water table • Assess the continuity of geologic strata • Locate fractures, faults, and buried bedrock channels

26. Seismic Refraction • Refraction is generally used in very shallow studies. (A few Hundred Feet) • With enough energy a study of few thousand feet can be conducted

27. Seismic Refraction

28. Seismic Reflection • Same technique as refraction but can measure much deeper.

29. Seismic Reflection

30. Micro Gravity • Measures the change in earth’s gravitational field caused by changes in density in the soil or rock.

31. Micro Gravity

32. Applicationsof GPM

33. Applications of GPM

34. Downhole Geophysical Measurements • Natural Gamma Log • Gamma-Gamma (Density) Log • Neutron-Neutron (Porosity) Log • Induction Log • Resistance Log • Spontaneous-Potential Log • Temperature Log • Fluid Conductivity Log • Caliper Log

35. Natural Gamma Log • Records the amount of natural gamma radiation emitted from a rock body • K 30 and daughter products from U and Th decay series. • Clays and shales concentrate these elements due to cation exchange and adsorption

36. Gamma-Gamma (Density) Log • Measures bulk density • Probe contains both a radiation source and a detector.

37. Neutron-Neutron (Porosity) Log • Measures relative moisture content above the water-table and porosity below the water-table • Uses similar methods as the Gamma-Gamma (Density) Log

38. Induction Log • An electromagnetic induction method to measure the electrical conductivity in open or PVC-case boreholes above or below the water-table • The electrical conductivity is a function of soil and rock type, porosity, permeability, and specific conductance of the pore fluids

39. Induction Log • Because the response of the log will be a function of the specific conductance of the pore fluids, it can be used to determine the presence of inorganic contamination

40. Resistance Log • Used mainly in geologic correlation and locating fractures or washout zones. • Only ran in a open hole in the saturated zone

41. Spontaneous-Potential Log • Measures the natural potential (millivolts) developed between the borehole fluids and the surrounding rock material. • Applications include • Characterizing lithology • Providing information on the geochemical oxidation-reduction conditions • Providing an indication of fluid flow

42. Temperature Log • Measures Temperature of the borehole fluid. • Can indicate zones of groundwater flow in an uncased borehole, by recording an increase or decrease in temperature.

43. Fluid Conductivity Log • Measures specific conductance of the borehole fluid. A temperature log must also be run to correct values.

44. Caliper Log • This log simply provides a borehole measurement. It will indicate any problems in the borehole diameter.

45. General Characteristics and Use of Downhole Geophysical Logs

46. Summary • As you can see there are many different types of GPM. • Knowing which method to apply in the correct manner can lead to a more accurate understanding of the hydrogeology at a much cheaper price and with less damage to the surface.